Process of recovering selenium



- 7 keep the solution as cool as pass the precipitate would occlude sulphuric'yao d and possibly come the black amorphous.

Patented Oct. 8, 1929 UNITED STATES PATENT OFFICE DANIEL L. OGDEN AND ROGER E. VALENTINE, 0F WOODBRIDGE, NEW JERSEY, AS'

$IGNORS TO UNITED STATESMETALS REFINING COMPANY, OF CARTERET, NEW JERSEY, A CORPORATION OF NEW JERSEY rnoonss or :ancovnnme summon No Drawing.

, This invention provides an improved method for the production of selenium (Se) from solutions containing this element, in the form of selenious acid or salts of selcnious acid. In the roasting and smelting of ores, mattes, flue dust and other metallurgical prodnets, and in the smelting of slimes resulting from the electrolytic refining of, metals, selenium when present is volatilized in the form of selenium dioxide, which is recovered by water absorption in v acid (H SeO and by condensation in the tines as selenium dioxide crystals, ally known as whiskers. Selenious acid is formed when these crystalsiare dissolved in water and when the soda slags, made during the smelting and refi ing of electrolytic slimes mentioned before, are leached with water. Up to the time of our invention selenium has been obtained from the selenious acid by adding hydrochloric acid (HClA'q) to the solution to keep it acid suiliciently to prevent the tellurium, which is usually present, from also precipitating and then passing sulphur dioxide (made by burning sulphur-bumstone) through this acidified solution to precipitate the selenium. vIt was necessary to possible and to SO through slowly, as otherwise the v causing trouble later on,

down as a purplish-black modification ofselenium which is very hard to wash free from acid and to dry. The red amorphous form of selenium produced when the conditions were right was then washed carefully by decantation until free from sulphuric and hydrochloric acids, after whichit was heated with live steam to bring to the boiling temperature whereby the red amorphous was changed to (During the washin serious losses of selenium often occurred.

This black selenium was then filtered off, dried, ground, screened and packed for shipment.

The present invention provides a simpler, more rapid, and more economical process, whereby the production of black amorphous selenium is greatly facilitated.

By the new method the sump pit solution Application filed May 9, 1928. Serial No. 276,499.

the form of selenious' colloquierward converting The black amorphous precip tate occurs in ing occluded acid that this crushed and ground in wet condition while or other selenium-bearing solution containing selenium in the form of selenious acid is kept hot while introducing SO (preferably commercial IiquidSO and without adding hydrochloric acid to the solution; thereby black amorphous selenium is precipitated in lumps or granules. When the reaction has been completed, the precipitate is repeatedly washed with water, either hot or cold, to remove any residual acid; the lumpy precipitate is then ground while wet, using for this purpose preferably a continuous ball or rod mill. The ground material is then washed, filtered, and dried. The dried product requires of it passing through a suitably fine screen, whereupon it may be'packed for shipment An important feature of dillerentiation from the former process is that the solution is worked hot instead of cold, and that no hydrochloric acid is added or is present, and the selenium is thus precipitated immediately in the black amorphous form instead of first producing red amorphous selenium and aftthis into the black form.

the form of lumps, balls, or masses containv and liquid, and cannot practicably be' immediately dried and round; instead, the new process requires black amorphous precipitate be only screening, nearly all washing out the liberated acid and other liquid impurities.

The process, as operated on a practical scale, is more completely described as follows:

The sump pit solution which has washed i the exhaust gases from the flue or tower contains selenium in the form of selenious acid and its salts, with a trace of selenic acid; or if the solution results from leaching of soda slag, it should be acidulated, preferably by adding sulphuric acid. Such solutions re quire ordinarily to be filtered to. make them ready for treatment. The solution is placed in a suitable tank and treated while hot, or if the solution has cooled it should be heated to about 180 F. andmaintained at approximately that temperature during treatment.

The tank should be provided with an agitaducting the process is given for the informator, whereby the solution may be kept in. agition of workers as to the best mode known to tation during treatment. us for practising the inventionyand the delVhen the tank has been filled with the filtailed steps described arenot to be taken as tered solution the agitator is started and soessential features thereof. 70

lution raised to the required temperature and e claim as our invention:

, sulphur dioxide passed into the solution so 1. The treatment of a hot selenium-bearing that the gas will bubble up through it. The solution byintroduction of sulphur dioxide heat of reaction will rapidly raise the temin absence of hydrochloric acid to precipitate I prature to 180 to 190 F. and maintain this selenium in black amorphous form; separat- 75 mtemperature without the aid of external heat. ing the solution; grinding the wet precipi- The addition of S0 is continued until the tate; and washing and drying the selenium. solution shows by chemical test that all of the 2. Treating a hot selenium-bearing soluselenium has been precipitated; thereupon tion with agitation and introduction of sulthe introduction of S0 is cut off and the agiphur dioxide to precipitate selenium inblack 5 tation stopped. The solution is then allowed amorphous form. to settle and the liquor is decanted from,the .;1;-: 3. Producing black amorphous selenium by precipitate. The precipitated selenium is direct precipitation from a hot solution conthen washed two or more times by adding wataining selenious acid or salts.

2O ter and agitation and decantation. The pre- 4. Producing selenium by direct precipitas5 ci-pitate is then washed over onto a gravity. tion from a solution containing selenious acid filter, from which the wet precipitate is of black amorphous selenium and continutransferred to a wet mill for grinding. A ously crushing and washing the precipitate to suitable grinding mill is a continuous feed remove occluded acid and soluble impurities.

and overflow mill, either a ball mill or rod 5. Treatin a hot selenium-bearing solu- 90 mill. Water is kept flowing through the mill, tion with agitation and introduction of sulthe overflow running onto a suction filter of phur dioxide to precipitate black amorphous standard design equipped for washing the filselenium with absence of hydrochloric acid. ter cake. This washing removes any soluble In witness whereof, we have hereunto impurities which may have dissolved in the signed our names. 95

acid occluded in the precipitate and released DANIEL L. OGDEN. V by the breaking up of the latter during grind- ROGER E. VALENTINE.

ing. The washed filter cake which is selenium is then removed to pans and placed in an oven for drying, which is conducted at a tem 100 perature not over 200 F. The dried selenium is then screened and packed for shipment. It is found that nearly all of the dried material will pass through a screen of 180-200 mesh.

The time required (using'agitator tanks holding about cubic feet) is about three to six hours forthe agitation with sulphur dioxide; subsequent washing in the agitator and emptying, three to four hours; grinding,

two to four hours; finall washing, about two hours; and drying and packing, 24 to 48 hours.

The selenium produced by this processv is 50 found to have a purity approximating 99 per cent. I

The new process has the advantages of more than doubling the capacity of existing plants without additional labor; the selenium product is as pure as that heretofore com- Inercially produced; the time required by the entire process is shortened to less than onehalf that formeily needed and the waste of selenium is greatly reduced due to hot pre- 60 cipitation and subsequent Wet grinding; the 12 use of hydrochloricacid (HClAq) is avoided, and less care and skill in the conduct of the process are required.

It is to be understood that the detailed de- 5 scription of the preferred manner of con- 

